Hot runner mold technology is directed at optimizing the flow of molten plastic material from the injection molding machine nozzle through the manifold and nozzle to the mold cavity. The molten plastic material leaving the machine nozzle is uniformly spread under high injection pressure throughout the manifold melt passages and then through the melt passage in the nozzle to the mold cavity of the article being molded therein. The number of melt channels and the number of nozzle assemblies are determined by either the number of desired gate areas or by the number of cavities to be filled. The melt channels are designed to evenly balance the flow of molten plastic and to fill the desired number of cavities in the mold at the same time. Furthermore, the manifolds and nozzles are heated by means of electric heating sources throughout the molding process to keep the melt temperature and pressure uniform once reaching the mold cavity of the molded article.
Demanding production requirements and demanding high quality of the molded article drives new ways to reduce maintenance costs. Maintenance costs are directly related to scheduled and unscheduled mold service during mold production runs. Scheduled mold service is a preventative maintenance schedule designed to either prolong the hot runner mold's life cycle or extend the use of the same mold for different application by means of replacing a few components. Unscheduled mold service is directly related to the unexpected mold downtime caused by the dynamic operating environment wherein the hot runner mold is subject to the high pressures, high temperatures and is used with abrasive and corrosive plastic materials. Thereupon, ways to maintain the hot runner mold in the machine are preferred, thus minimizing the downtime and maximizing the run time.
The nozzle design must be optimized to permit uniform temperature distribution along the nozzle body for equal cavity filling and improved molded article quality. The design of hot runner nozzles is particularly problematic in inside center gate applications. Inside center gating is used for cup-shaped plastic molded articles which must not have any external gate marks, or where the top surface of the product will be printed or a label applied. External gate marks can easily perforate the label, or may not be very appealing on the products, especially in the cosmetic industry. The design of hot runner nozzles for the inside center gate molding of small sized articles is particularly challenging since the nozzle size must be limited. However, limiting the nozzle size (diameter) decreases the mass of the nozzle thereby decreasing the heat conductivity of the nozzle which may result in nozzle freeze up.
There is a need for a nozzle that can be easily serviced during the scheduled and unscheduled servicing of the hot runner mold. At the same time there is a need for a smaller diameter and/or extended nozzle for inside center gate applications or other applications, both having uniform temperature distribution along the nozzle body extended to gate. Furthermore, there is also a need for a nozzle design suitable for use in color change applications which accommodates a reduced volume of accumulated plastic around the nozzle tip.